feat(chat): LLM failover chain + show which model answered

Chat is LLM-essential but not model-specific — instead of failing when the
pinned model is down/drifted, it now fails over across an ordered chain and
reports which model actually answered.

- Ordered fallback: an Llm declares `extraConfig.fallbacks: string[]`; the
  dispatcher builds primary-pool → fallback-pool(s) candidates and tries them
  in order (resolveCandidatesWithFallbacks).
- Fail over on real failures, not just transport: runOneInference now advances
  on a non-2xx status (e.g. a 400 from a drifted model) or an empty/invalid
  completion, not only thrown transport errors. Streaming fails over
  pre-first-chunk (already threw on 4xx).
- Transparency: ChatResult + the SSE `final` frame carry {llm, model,
  failedOver}; the CLI prints `model: <llm> (<model>)` each turn and
  `⚠ failed over → answered by …` when a fallback was used.
- Exhaustion names the last model + upstream body (not "no choice").

Tests: 3 failover unit tests (primary 400 → fallback answers + model reported;
primary answers → failedOver=false; all fail → clear aggregated error).
mcpd 948 + CLI 508 green; tsc + lint clean. docs/reliability.md updated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/reliability.md

@@ -28,10 +28,24 @@ Note: the gate's prompt-ranking uses the **heavy client provider's own model**
 it deliberately does *not* force the project's vLLM model onto it (doing so made
 every selection fail silently when the model wasn't anthropic-servable).
 
-## LLM-*essential* operations
+## LLM-*essential* operations — failover chain
 
-Chat needs the LLM — it can't fall back. It must **fail fast with a clear,
+Chat needs *an* LLM but not a *specific* one. Instead of failing when the pinned
-actionable message** instead of a vague one. Chat surfaces the upstream status +
+model is down, chat **fails over across an ordered chain** and **reports which
-body (which names the model + reason), e.g.
+model actually answered**.
-`LLM returned no completion (HTTP 400): {"error":"model deepseek-v4-flash not found"}`,
+
-rather than "Adapter returned no choice".
+- **Chain:** an `Llm` declares fallbacks in `extraConfig.fallbacks: string[]`
+  (Llm names, in order). The dispatcher builds an ordered candidate list —
+  the primary's pool, then each fallback's pool — and tries them in order.
+- **Fails over on real failures**, not just transport: a non-2xx status
+  (e.g. a drifted model's `400`) or an empty/invalid completion now advances to
+  the next candidate (`chat.service.ts` `runOneInference`). Streaming fails over
+  pre-first-chunk.
+- **Transparency:** `ChatResult` / the SSE `final` frame carry `llm`, `model`,
+  and `failedOver`. The CLI prints `model: <llm> (<model>)` per turn, and
+  `⚠ failed over → answered by <llm> (<model>)` when a fallback was used.
+- **Exhaustion is clear:** if every candidate fails, the error names the last
+  model + upstream status/body (not "no choice").
+
+Homelab chain: `vllm-current` (the served vLLM) → an `anthropic-fallback` server
+Llm as the always-up last resort.

src/cli/src/commands/chat.ts

@@ -198,7 +198,8 @@ async function runOneShot(
     // `thread: <id>` — single space after the colon, matching the streaming
     // path (line 160 below) so any tooling/regex that watches one form picks
     // up the other too.
-    process.stderr.write(styleStats(`(${String(words)}w · ${(words / sec).toFixed(1)} w/s · ${sec.toFixed(1)}s)`) + `  thread: ${res.threadId}\n`);
+    const answered = formatAnswered(res.llm, res.model, res.failedOver);
+    process.stderr.write(styleStats(`(${String(words)}w · ${(words / sec).toFixed(1)} w/s · ${sec.toFixed(1)}s)`) + `  thread: ${res.threadId}${answered !== '' ? `  ${answered}` : ''}\n`);
     return;
   }
   const bar = installStatusBar();
@@ -258,6 +259,8 @@ async function runRepl(
           const res = await chatRequestNonStream(deps, subject, body);
           threadId = res.threadId;
           process.stdout.write(`${res.assistant}\n`);
+          const answered = formatAnswered(res.llm, res.model, res.failedOver);
+          if (answered !== '') process.stderr.write(`${styleStats(`(${answered})`)}\n`);
         } else {
           threadId = await streamOnce(deps, subject, line, threadId, overrides, bar);
           process.stdout.write('\n');
@@ -455,7 +458,7 @@ async function chatRequestNonStream(
   deps: ChatCommandDeps,
   subject: ChatSubject,
   body: Record<string, unknown>,
-): Promise<{ assistant: string; threadId: string; turnIndex: number }> {
+): Promise<{ assistant: string; threadId: string; turnIndex: number; llm?: string; model?: string; failedOver?: boolean }> {
   const url = new URL(`${deps.baseUrl}/api/v1/${subject.path}/chat`);
   const payload = JSON.stringify(body);
   return new Promise((resolve, reject) => {
@@ -482,7 +485,7 @@ async function chatRequestNonStream(
           return;
         }
         try {
-          resolve(JSON.parse(raw) as { assistant: string; threadId: string; turnIndex: number });
+          resolve(JSON.parse(raw) as { assistant: string; threadId: string; turnIndex: number; llm?: string; model?: string; failedOver?: boolean });
         } catch (err) {
           reject(new Error(`malformed response: ${(err as Error).message}`));
         }
@@ -550,6 +553,7 @@ async function streamOnce(
       }
       let buf = '';
       let resolvedThread = threadId ?? '';
+      let answered = '';
       res.setEncoding('utf-8');
       res.on('data', (chunk: string) => {
         buf += chunk;
@@ -590,6 +594,7 @@ async function streamOnce(
                   break;
                 case 'final':
                   if (evt.threadId !== undefined) resolvedThread = evt.threadId;
+                  answered = formatAnswered(evt.llm, evt.model, evt.failedOver);
                   break;
                 case 'error':
                   process.stderr.write(`\n[error: ${evt.message ?? ''}]\n`);
@@ -606,10 +611,11 @@ async function streamOnce(
         const final = formatStats(stats, false);
         // Inline final-stats footer (permanent record): goes through the
         // scroll region, so a copy-pasted transcript captures it.
-        if (final !== '' && STDERR_IS_TTY) {
+        const footer = [final, answered].filter((s) => s !== '').join('  ·  ');
-          process.stderr.write(`\n${styleStats(`(${final})`)}`);
+        if (footer !== '' && STDERR_IS_TTY) {
-        } else if (final !== '') {
+          process.stderr.write(`\n${styleStats(`(${footer})`)}`);
-          process.stderr.write(`\n(${final})`);
+        } else if (footer !== '') {
+          process.stderr.write(`\n(${footer})`);
         }
         // Live status bar: pin the final value so it stays visible between
         // turns (answers "how fast was the last one?").
@@ -637,6 +643,16 @@ interface ChatStreamFrame {
   threadId?: string;
   turnIndex?: number;
   message?: string;
+  llm?: string;
+  model?: string;
+  failedOver?: boolean;
+}
+
+/** One-line "which model answered" footer, dim; loud when it failed over. */
+function formatAnswered(llm?: string, model?: string, failedOver?: boolean): string {
+  if (llm === undefined && model === undefined) return '';
+  const label = model !== undefined && model !== '' ? `${llm ?? '?'} (${model})` : (llm ?? '?');
+  return failedOver === true ? `⚠ failed over → answered by ${label}` : `model: ${label}`;
 }
 
 // ANSI codes for the reasoning sidebar. Dim + italic visually separates

src/mcpd/src/services/chat.service.ts

@@ -111,6 +111,10 @@ export interface ChatStreamChunk {
   threadId?: string;
   turnIndex?: number;
   message?: string;
+  /** On `final`: which Llm/model produced the reply, and whether it failed over. */
+  llm?: string;
+  model?: string;
+  failedOver?: boolean;
 }
 
 /**
@@ -151,6 +155,12 @@ export interface ChatResult {
   threadId: string;
   assistant: string;
   turnIndex: number;
+  /** Which Llm actually produced the reply (after any failover). */
+  llm: string;
+  /** The model string that answered (e.g. 'qwen3-thinking' or 'claude-opus-4-...'). */
+  model: string;
+  /** True when the reply came from a fallback rather than the primary Llm. */
+  failedOver: boolean;
 }
 
 /**
@@ -173,7 +183,12 @@ interface PreparedChatContext {
   threadId: string;
   history: OpenAiMessage[];
   systemBlock: string;
+  /** Ordered dispatch list: the primary Llm's pool first, then each fallback
+   * Llm's pool (from the primary's extraConfig.fallbacks). The loop tries them
+   * in order and fails over on error/4xx/no-completion. */
   poolCandidates: PoolCandidate[];
+  /** Name of the intended primary Llm (to detect + report a failover). */
+  primaryLlm: string;
   mergedParams: AgentChatParams;
   toolList: ChatTool[];
   projectId: string | null;
@@ -249,9 +264,11 @@ export class ChatService {
   private async runChatLoop(ctx: PreparedChatContext): Promise<ChatResult> {
     let assistantFinal = '';
     let lastTurnIndex = ctx.startingTurnIndex;
+    let winner: PoolCandidate | undefined;
     try {
       for (let i = 0; i < ctx.maxIterations; i += 1) {
         const result = await this.runOneInference(ctx);
+        winner = result.candidate;
         const choice = extractChoice(result.body);
         if (choice === null) {
           // Surface the upstream body (LiteLLM/vLLM names the model + reason,
@@ -312,7 +329,14 @@ export class ChatService {
         assistantFinal = assistantText;
         lastTurnIndex = finalMsg.turnIndex;
         await this.chatRepo.touchThread(ctx.threadId);
-        return { threadId: ctx.threadId, assistant: assistantFinal, turnIndex: lastTurnIndex };
+        return {
+          threadId: ctx.threadId,
+          assistant: assistantFinal,
+          turnIndex: lastTurnIndex,
+          llm: winner?.llmName ?? ctx.primaryLlm,
+          model: winner?.modelOverride ?? '',
+          failedOver: winner !== undefined && winner.llmName !== ctx.primaryLlm,
+        };
       }
       throw new Error(`Chat loop exceeded ${String(ctx.maxIterations)} iterations without a terminal turn`);
     } catch (err) {
@@ -333,6 +357,7 @@ export class ChatService {
 
   /** Shared streaming turn loop. Persists rows in lockstep. */
   private async *runChatStreamLoop(ctx: PreparedChatContext): AsyncGenerator<ChatStreamChunk> {
+    const winnerRef: { candidate: PoolCandidate | null } = { candidate: null };
     try {
       for (let i = 0; i < ctx.maxIterations; i += 1) {
         // `reasoning` is accumulated alongside `content` so we can fall back
@@ -348,7 +373,7 @@ export class ChatService {
           toolCalls: [],
         };
         let finishReason: string | null = null;
-        for await (const chunk of this.streamInference(ctx)) {
+        for await (const chunk of this.streamInference(ctx, winnerRef)) {
           if (chunk.done === true) break;
           if (chunk.data === '[DONE]') break;
           const evt = parseStreamingChunk(chunk.data);
@@ -452,7 +477,14 @@ export class ChatService {
           content: persistedContent,
         });
         await this.chatRepo.touchThread(ctx.threadId);
-        yield { type: 'final', threadId: ctx.threadId, turnIndex: finalMsg.turnIndex };
+        yield {
+          type: 'final',
+          threadId: ctx.threadId,
+          turnIndex: finalMsg.turnIndex,
+          llm: winnerRef.candidate?.llmName ?? ctx.primaryLlm,
+          model: winnerRef.candidate?.modelOverride ?? '',
+          failedOver: winnerRef.candidate !== null && winnerRef.candidate.llmName !== ctx.primaryLlm,
+        };
         return;
       }
       throw new Error(`Chat loop exceeded ${String(ctx.maxIterations)} iterations without a terminal turn`);
@@ -482,10 +514,14 @@ export class ChatService {
     systemBlock: string;
     toolList: ChatTool[];
     mergedParams: AgentChatParams;
-  }): AsyncGenerator<{ data: string; done?: boolean }> {
+  }, winnerRef?: { candidate: PoolCandidate | null }): AsyncGenerator<{ data: string; done?: boolean }> {
     let lastErr: Error | null = null;
     for (const c of ctx.poolCandidates) {
       try {
+        // Record the candidate we're committing to; if it throws before its
+        // first chunk the loop overwrites this on the next iteration, so after
+        // a successful stream this holds the Llm that actually answered.
+        if (winnerRef) winnerRef.candidate = c;
         yield* this.streamOneCandidate(c, ctx);
         return;
       } catch (err) {
@@ -585,17 +621,26 @@ export class ChatService {
     systemBlock: string;
     toolList: ChatTool[];
     mergedParams: AgentChatParams;
-  }): Promise<{ status: number; body: unknown }> {
+  }): Promise<{ status: number; body: unknown; candidate: PoolCandidate }> {
     let lastErr: Error | null = null;
     for (const c of ctx.poolCandidates) {
       try {
-        return await this.dispatchOneCandidate(c, ctx);
+        const result = await this.dispatchOneCandidate(c, ctx);
+        // Fail over on an upstream error status or an empty/invalid completion —
+        // NOT just transport errors. This is what lets a 400 (drifted/down model)
+        // fall through to the next candidate and, ultimately, the anthropic fallback.
+        if (result.status >= 400 || extractChoice(result.body) === null) {
+          const detail = typeof result.body === 'string' ? result.body : JSON.stringify(result.body);
+          lastErr = new Error(`LLM '${c.llmName}' (model '${c.modelOverride}') HTTP ${String(result.status)}: ${detail.slice(0, 300)}`);
+          continue;
+        }
+        return { ...result, candidate: c };
       } catch (err) {
-        lastErr = err as Error;
+        lastErr = new Error(`LLM '${c.llmName}' (model '${c.modelOverride}'): ${(err as Error).message}`);
-        // Try the next pool member.
+        // Try the next candidate.
       }
     }
-    throw lastErr ?? new Error('chat dispatch exhausted: no pool candidates');
+    throw lastErr ?? new Error('chat dispatch exhausted: no candidates');
   }
 
   private async dispatchOneCandidate(
@@ -634,7 +679,7 @@ export class ChatService {
   private async prepareContext(args: ChatRequestArgs): Promise<PreparedChatContext> {
     const agent = await this.agents.getByName(args.agentName);
     const pinned = await this.llms.getByName(agent.llm.name);
-    const poolCandidates = await this.resolvePoolCandidates(pinned);
+    const poolCandidates = await this.resolveCandidatesWithFallbacks(pinned);
     if (poolCandidates.length === 0) {
       // Pool exists but every member is inactive (publishers offline,
       // public Llm explicitly disabled, etc.). Surface a clear error
@@ -713,6 +758,7 @@ export class ChatService {
       history,
       systemBlock,
       poolCandidates,
+      primaryLlm: pinned.name,
       mergedParams,
       toolList: filteredTools,
       projectId,
@@ -774,7 +820,7 @@ export class ChatService {
     }
     const pinned = await this.llms.getByName(project.llmProvider);
     // project.llmModel overrides the pinned Llm's own model for this chat.
-    const poolCandidates = await this.resolvePoolCandidates(pinned, project.llmModel ?? undefined);
+    const poolCandidates = await this.resolveCandidatesWithFallbacks(pinned, project.llmModel ?? undefined);
     if (poolCandidates.length === 0) {
       throw new NotFoundError(
         `No active Llm in pool '${effectivePoolName(pinned)}' (project '${project.name}' → ${pinned.name})`,
@@ -824,6 +870,7 @@ export class ChatService {
       history,
       systemBlock,
       poolCandidates,
+      primaryLlm: pinned.name,
       mergedParams,
       toolList: filteredTools,
       projectId,
@@ -892,6 +939,30 @@ export class ChatService {
     }));
   }
 
+  /**
+   * Build the ordered dispatch list: the primary Llm's pool, then each fallback
+   * Llm's pool (declared on the primary via `extraConfig.fallbacks: string[]`).
+   * The dispatch loop tries them in order and fails over on error/4xx/no-completion.
+   * A missing/misconfigured fallback is skipped, never fatal.
+   */
+  private async resolveCandidatesWithFallbacks(
+    pinned: { name: string; poolName: string | null; extraConfig?: unknown },
+    modelOverride?: string,
+  ): Promise<PoolCandidate[]> {
+    const candidates = await this.resolvePoolCandidates(pinned, modelOverride);
+    for (const name of extractFallbackNames(pinned.extraConfig)) {
+      if (name === pinned.name) continue; // no self-reference loops
+      try {
+        const fb = await this.llms.getByName(name);
+        // A fallback uses its OWN configured model — not the primary's override.
+        candidates.push(...(await this.resolvePoolCandidates(fb)));
+      } catch {
+        // Fallback missing/inactive — skip it, keep the rest of the chain.
+      }
+    }
+    return candidates;
+  }
+
   /**
    * Resolves a personality (request override → agent default) and returns
    * its bound prompt contents in `PersonalityPrompt.priority` desc order.
@@ -1099,6 +1170,14 @@ function safeParseJson(s: string): unknown {
   }
 }
 
+/** Read an ordered fallback-Llm list from a primary Llm's `extraConfig.fallbacks`. */
+function extractFallbackNames(extraConfig: unknown): string[] {
+  if (extraConfig === null || typeof extraConfig !== 'object') return [];
+  const raw = (extraConfig as { fallbacks?: unknown }).fallbacks;
+  if (!Array.isArray(raw)) return [];
+  return raw.filter((x): x is string => typeof x === 'string' && x.length > 0);
+}
+
 /** The gated virtual secret-read tool offered to project chat with --allow-secrets. */
 function secretReadTool(): ChatTool {
   return {

src/mcpd/tests/chat-service.test.ts

@@ -1080,3 +1080,64 @@ function mockPersonalityRepo(
     findBinding: vi.fn(async () => null),
   };
 }
+
+// ── Failover chain + "which model answered" reporting ──
+describe('ChatService — failover chain', () => {
+  const NOW2 = new Date();
+  function llmRow(name: string, model: string, fallbacks: string[] = []): Record<string, unknown> {
+    return {
+      id: `id-${name}`, name, type: 'openai', model, url: '', tier: 'fast', description: '',
+      apiKeySecretId: null, apiKeySecretKey: null,
+      extraConfig: fallbacks.length > 0 ? { fallbacks } : {},
+      poolName: null, kind: 'public', providerSessionId: null, status: 'active',
+      lastHeartbeatAt: null, inactiveSince: null, version: 1, createdAt: NOW2, updatedAt: NOW2,
+    };
+  }
+  function failoverLlms(): LlmService {
+    const rows: Record<string, Record<string, unknown>> = {
+      'primary-llm': llmRow('primary-llm', 'primary-model', ['fallback-llm']),
+      'fallback-llm': llmRow('fallback-llm', 'fallback-model'),
+    };
+    return {
+      getByName: vi.fn(async (name: string) => ({ ...rows[name], apiKeyRef: null })),
+      findByPoolName: vi.fn(async (pool: string) => [rows[pool]]),
+      resolveApiKey: vi.fn(async () => 'k'),
+    } as unknown as LlmService;
+  }
+  function agentPinnedTo(llmName: string): AgentService {
+    return {
+      getByName: vi.fn(async (name: string) => ({
+        id: `agent-${name}`, name, description: '', systemPrompt: 'sys',
+        llm: { id: 'x', name: llmName }, project: null, defaultPersonality: null,
+        proxyModelName: null, defaultParams: {}, extras: {}, ownerId: 'o', version: 1, createdAt: NOW2, updatedAt: NOW2,
+      })),
+    } as unknown as AgentService;
+  }
+  const status400 = (): NonStreamingResult => ({ status: 400, body: { error: 'model not served' } });
+
+  it('fails over to the fallback Llm on a primary 400 and reports the answering model', async () => {
+    const adapter = scriptedAdapter([status400(), chatCompletion('fallback answer')]);
+    const svc = new ChatService(agentPinnedTo('primary-llm'), failoverLlms(), adapterRegistry(adapter), mockChatRepo(), mockPromptRepo(), mockTools());
+    const res = await svc.chat({ agentName: 'a', userMessage: 'hi', ownerId: 'o' });
+    expect(res.assistant).toBe('fallback answer');
+    expect(res.failedOver).toBe(true);
+    expect(res.llm).toBe('fallback-llm');
+    expect(res.model).toBe('fallback-model');
+  });
+
+  it('reports the primary model + failedOver=false when the primary answers', async () => {
+    const adapter = scriptedAdapter([chatCompletion('primary answer')]);
+    const svc = new ChatService(agentPinnedTo('primary-llm'), failoverLlms(), adapterRegistry(adapter), mockChatRepo(), mockPromptRepo(), mockTools());
+    const res = await svc.chat({ agentName: 'a', userMessage: 'hi', ownerId: 'o' });
+    expect(res.assistant).toBe('primary answer');
+    expect(res.failedOver).toBe(false);
+    expect(res.llm).toBe('primary-llm');
+    expect(res.model).toBe('primary-model');
+  });
+
+  it('throws a clear aggregated error naming the model when every candidate fails', async () => {
+    const adapter = scriptedAdapter([status400(), status400()]);
+    const svc = new ChatService(agentPinnedTo('primary-llm'), failoverLlms(), adapterRegistry(adapter), mockChatRepo(), mockPromptRepo(), mockTools());
+    await expect(svc.chat({ agentName: 'a', userMessage: 'hi', ownerId: 'o' })).rejects.toThrow(/HTTP 400/);
+  });
+});